The present invention, in some embodiments thereof, relates to an optical frequency converter for use in optical conversion and, more particularly, but not exclusively, to efficient broadband optical conversion.
The generation of tunable frequency optical radiation typically relies on nonlinear frequency conversion in crystals. In this process, light of two frequencies or two colors is introduced into the nonlinear crystal, resulting in the generation of a third color with their sum or difference frequency. These processes, which also known as frequency up-conversion or frequency down-conversion are typically very sensitive to the incoming frequencies, in such a manner as to require angle, temperature or other tuning mechanisms in order to support efficient frequency conversion. This difficulty is of particular importance when trying to efficiently convert broadband frequency optical signals, since simultaneous phase matching of a broad frequency range is difficult.
Currently, most efficient frequency conversion devices rely on a single nonlinear crystal, which is either temperature or angle tuned to enhance efficiency. Typically, this results only in a narrow spectral band that is efficiently converted. Quasi-Phase Matching (QPM), in which a nonlinear crystal is modified periodically, results in improved efficiencies, but still within a narrow predetermined band. Segmented periodic structures [1] or aperiodic quasi phase matching [2] have been shown to improve the bandwidth response, but at a cost of a significantly reduced efficiency. Fejer and co-worker have used aperiodic QPM structures for chirp tuning of second harmonic generation (SHG) signal, generated by ultrashort pulses. They demonstrated that by using such a structure, not only is the SHG signal enhanced, but also the temporal chirp gain in the process can be controlled [3]. A recent structure, used by Baudrier-Raybaut and co-worker, was a totally disordered material (Random quasi-phase-matching), which lead to an extremely loose frequency selectivity and caused an inversion in a wide range of frequencies, though again it had a very low efficiency [4]. Thus broadband frequency conversion was achieved but with very low efficiency. To date as far as we know, broadband frequency converters have proven to be inefficient, and efficient converters are narrowband. The combination of broadband and efficient conversion is currently not known.
Additional background art includes    [1] K. Mizuuchi, K. Yamamoto, M. Kato, and H. Sato Broadening of the Phase-Matching Bandwidth in Quasi-Phased-Matched Second Harmonic Generation, IEEE Journal of Quantum Electronics 30 (7), 15961604 (1994).    [2] M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances, IEEE Journal of Quantum Electronics 28 (11), 2631-2654 (1992); M. L. Bortz, M. Fujimura, and M. M. Fejer, Increased acceptance bandwidth for quasi-phasematched second harmonic generation in LiNbO3 waveguides, Electronics Letters 30 (1), 34-35 (1994).    [3] M. A. Arbore, A. Galvanauskas, D. Harter, M. H. Chou, and M. M. Fejer, Engineerable compression of ultrashort pulses by use of second-harmonic generation in chirped-period-poled lithium niobate, Optics Letters 22 (17), 13411343 (1997).    [4] M. Baudrier-Raybaut, R. Haidar, Ph. Kupecek, Ph. Lemasson, and E. Rosencher, Random quasi phase matching in bulk polycrystalline isotropic nonlinear materials, Nature 432 (7015), 374-376 (2004).    [5] R. W. Boyd, Nonlinear Optics (Academic Press, 2005) pages 79-83.    [6] A. Messiah, Quantum Mechanics Vol. II (Wiley, 1963) pages 739-759.